On Tuesday, October 1, 2019 at 3:44:11 PM UTC-4, RR wrote:

The legalitys are not so clear [...]

FAR 23.1447 spells it out (for certificated aircraft). Above 18K the requirement is for a mask that covers nose and mouth.

The fact that some people can get away with a cannula at 26K (once) does not mean that this is a sound practice. We've had two hypoxia incidents (that I know of) at Mt Washington due to pilots emulating certain old timers. Physiology varies. Physical fitness doesn't seem to be a factor (in fact casual observation suggests the reverse :-)).

T8